Field
The presently disclosed subject relates to a driver for an optical deflector using a combined (synthesized) saw-tooth drive voltage and a method for controlling the optical deflector. The optical deflector can be applied as an optical scanner to a laser pico projector, a laser radar, a bar code reader, an area sensor, a head lamp, a head-up display unit, and other optical apparatuses, to generate scanning light.
Description of the Related Art
Generally, in an optical scanner or the like, an optical deflector is constructed by a micro electro mechanical system (MEMS) device manufactured by using semiconductor manufacturing processes and micro machine technology.
A first prior art optical deflector as a two-dimensional MEMS device is constructed by a mirror, a pair of torsion bars coupled to the mirror along an axis (X-axis), an inner frame (movable frame) surrounding the mirror and the torsion bars, inner piezoelectric actuators coupled between the torsion bars and supported by the inner frame via inner coupling portions, serving as cantilevers for rocking the mirror with respect to the X-axis of the mirror, an outer frame (fixed frame) surrounding the inner frame, and outer piezoelectric actuators coupled between the inner frame and the outer frame, serving as cantilevers for rocking the mirror along another axis (Y-axis) of the mirror (see: JP2009-223165A).
Generally, the inner piezoelectric actuators are driven by a relatively high frequency such as 20 kHz for a horizontal scanning, while the outer piezoelectric actuators are driven by a relatively low frequency such as 60 Hz for a vertical scanning. For example, the inner piezoelectric actuators rock the mirror through the torsion bars, so that the inner piezoelectric actuators are driven by two synchronous sinusoidal-wave drive voltages. On the other hand, the outer piezoelectric actuators rock the mirror without torsion bars, so that the outer piezoelectric actuators are driven by two synchronous saw-tooth drive voltages which have a ratio of arising period (falling period) to a falling period (rising period) of 9:1 so as to exhibit longer linearly-changed drive voltages.
However, the synchronous saw-tooth drive voltages include harmonic frequency components in addition to their fundamental frequency such as 60 Hz. Therefore, even when the fundamental frequency of the synchronous saw-tooth drive voltages is smaller than a main natural (resonant) frequency Fr such as 1080 Hz and a pumping natural (resonant) frequency Fp such as 790 Hz of a mechanically-vibrating system of the mirror depending upon the structure of the outer piezoelectric actuators, the fundamental frequency would be superposed onto the above-mentioned harmonic frequency components. Note that the main natural frequency Fr is with respect to the Y-axis of the mirror, while the pumping natural frequency is with respect to a Z-axis perpendicular to the X-axis and the Y-axis. Therefore, the mirror would fluctuate due to the higher-order harmonic frequencies of the main natural frequency Fr and the pumping natural frequency Fp. As a result, a higher frequency vibration would be superimposed onto the rocking of the mirror, so that the scanning speed would fluctuate to cause distortion of an image.
A second prior art optical deflector as a one-dimensional MEMS device is constructed by a mirror, a supporting member, an oscillation axis coupled between the supporting member and the mirror, and oscillation members coupled to the oscillation axis (see: US2008/0297869A1 & JP2008-299297). A driver synthesizes a plurality of rectangular pulse voltages corresponding to a plurality of natural frequencies to form a rectangular pulse drive voltage which may be a saw-tooth or triangular wave drive voltage (see: paragraph 0036 of US2008/0297869A1 & paragraph 0014 of JP2008-299297).
Even in the above-described second prior art optical deflector, when the saw-tooth wave drive voltage synthesized by the plurality of natural frequencies is applied to the oscillation members, the mirror would fluctuate due to the harmonic components of the natural frequencies such as Fr and Fp.
A third prior art optical deflector as a two-dimensional MEMS device is constructed by a mirror, an inner frame (movable frame) surrounding the mirror through a pair of torsion bars, an outer frame (fixed frame), and four piezoelectric actuators fixed between the inner frame and serving as cantilevers for two-dimensionally rocking the mirror along the X-axis (horizontal direction) and the Y-axis (vertical direction) (see: FIG. 2 of US2008/0239252A and JP2008-249797A). Even in this third prior art optical deflector, the piezoelectric actuators are driven by a sinusoidal-wave voltage for a horizontal scanning, and the piezoelectric actuators are driven by a saw-tooth wave voltage for a vertical scanning. In the latter scanning, in order to correct distortion in an image along the Y-axis (vertical direction), a harmonic suppressing unit is provided to remove the higher harmonic components from the saw-tooth drive voltages for the vertical direction to obtain non-linear saw-tooth drive voltages for a vertical direction to be applied to the piezoelectric actuators (see: FIGS. 6C and 7 of US2008/0239252A and JP2008-249797A). For example, the range of the order of higher harmonic components to be removed may be 10 times or larger than the fundamental frequency (see: paragraph 147 of US2008/0239252A & paragraph 118 of JP2008-249797A).
In the above-described third prior art optical deflector, if the fundamental frequency of the vertical direction is 60 Hz, the harmonic components higher than 600 Hz are removed from the saw-tooth drive voltages for a vertical direction. However, the distortion along the Y-axis (in the vertical direction) is actually caused by higher-order harmonic components such as 540 Hz, 360 Hz, . . . of the main natural frequency Fr=1080 Hz and higher-order harmonic components such as 395 Hz, 263 Hz, . . . of the pumping natural frequency Fp=790 Hz. Therefore, it is impossible to completely remove the fluctuations.
A fourth prior art optical deflector as a two-dimensional MEMS device is constructed by a mirror, an inner frame coupled via inner torsion bars to the mirror, and an outer frame coupled via outer torsion bars to the inner frame. The mirror is electromagnetically driven by a sinusoidal-wave drive voltage having a relatively high frequency, and also, is electromagnetically driven by a saw-tooth wave drive voltage having a relatively low frequency such as 60 Hz. In this case, the saw-tooth wave drive voltage is synthesized with another saw-tooth wave drive voltage by shifting a phase of 1/(2·Fr) where Fr is a natural (resonant) frequency, so that unnecessary oscillations caused by the natural frequency are suppressed (see: US2015/0043047A1 & JP2013-171226A).
In the above-described fourth prior art optical deflector, however, the fluctuations along the Y-axis (in the vertical direction) are actually caused by higher-order harmonic components such as 540 Hz, 360 Hz, . . . of the main natural frequency Fr=1080 Hz and higher-order harmonic components such as 395 Hz, 263 Hz, . . . of the pumping natural frequency Fp=790 Hz. Therefore, it is impossible to completely remove the fluctuations from the vibration of the mirror.